When my very first siege came (40d), I was terrified. I sealed the underground section of the fort.
I had, of course, placed workshops I thought might get smoky, stinky, or noisy outside in the open air. This was almost all of them. I had also placed my eating area outside, because I liked the idea of drunken summer picnics in the warm jungle. And there was a sleeping area out there, because I liked giving the dwarves the option to camp under the stars.
The ground had gotten muddy under the outdoor beds pretty quickly. I thought this was a serious problem for some reason. So, for the first couple years, much of the fortress' production was focused on finding a way to solve this problem. When the siege finally came, I had recently placed a constructed floor to keep things cleaner under the outdoor beds. This was Angelworks' most outstanding project.
Underground, I only placed things that I did not want to get wet in the rain. So it was the treasure, and some of the food, and some offices. I placed only one bed down there, assuming only an occasional sick dwarf would want to sleep in the stinky, dark cave. (The survivors of the siege wound up fighting over that bed for many years to come.) There wasn't much down there.
In other words, *Angelworks* was built without a proper thought toward security.
So I watched with horror as the evil siegers dismembered several helpless "camping" dwarves, and the many carpenters and furnace operators and cooks outdoors. I cried for help. The more experienced player who had taught me the controls came over and was all, "Why did you put all those workshops outside?" and, "What the heck, you built all the beds...on a constructed floor...*outside*
", and, most importantly, "Where are the traps I told you to build?!"
"I built lots of traps like you said!" I replied. I had set quite a lot of little animal traps with gems and food in them. I was very dubious about their effectiveness, as all I'd seen happen with them in the first few years, was animals coming up and stealing the bait. But I figured they were supposed to catch goblins when the time was right so... Well, wrong kind of trap, it turned out.
I think my total obsession with keeping every dwarf at 100% full-time employment (unwittingly giving them no time to make friends) was what prevented the tantrum spiral after 75% of my population died. I think the siegers suffered only one casualty while annihilating my outdoor population. Naturally, I kept the underground part of the fort sealed while they hung around for the next six years or so.
Then I accidentally flooded the fortress...and then also the world somehow...while trying to build a well (we were out of booze the whole time...the seeds were outside). There was a lot of death (including the siegers, hooray) and I think at that point I abandoned it and ran for my life. My subsequent fortresses had a lot of the right kind of trap.
It sounds like you got a much more informed start than I did